A bit of musicological detective work this morning: in a library in Nantes, a previously unknown manuscript of original music by Mozart has been discovered. The city's vice-mayor is understandably proud – if they chose to sell this single sheet, with its nine staves (there would have been 12 originally; you can tell that the top third of this sheet is missing) they would make around £50,000.

Actually, it's not completely true to say this music has never been seen by the eyes of history: apart from Mozart's own handwriting, there's a scrawl by 19th-century German autograph-hunter Aloys Fuchs at the bottom of the page, who verified the hand as Mozart's. Based on my own admittedly limited researches – ie, looking at the facsimiles I'm lucky enough to have at home, of the Jupiter Symphony and Requiem, and comparing the handwriting – I'd say it's almost certain that Mozart really did write this.

What's fascinating about this sheet of manuscript is not what light it sheds on Mozart's existing masterpieces, but rather that it joins the hundred or so strong catalogue of unfinished drafts by Mozart. Unlike the legend, the real Wolfgang didn't always take musical dictation from God. Instead, he tried out ideas, rejecting some along the way, experimenting with his material until he found the right notes that would make the composition flow. Much of this working, there's no doubt, was done in his head or at the piano, so what makes this document so precious is that it is a physical reminder of Mozart's compositional humanity. What's more, it probably dates from Mozart's last years (the watermark suggests somewhere between 1787 and 1791, the year of his death).

So what have we got? Intriguingly, these are sketches for a religious work, which would put it in the category of the late works, like the Requiem, which Mozart composed to fuse his research into baroque music with his own, forward-looking ideas. There are four lines of vocal music, possibly a D minor-ish line in the bass clef - although it could be written in the soprano clef (which means that you read the bottom line of the stave as middle C, rather than E as you would do if were the treble clef). What I can make out of Mozart's accidentals on the photograph doesn't make it completely clear. There's also a line and half of D major, treble-clef instrumental music, marked 'Credo'. Was this part of a projected Mass setting, which would make it just a tiny fragment of a much larger conception? Or a smaller, self-contained work written for a potential patron? We shall probably never know; meanwhile, it's a reminder that even musical genius needs a reject pile.